believe me when I tell you I am about 5 minutes more experienced in aluminum welding than you...
3mm is .118" or just under 1/8" for us retarded 'mericans that insist on inches. I used 0.090" or 2.286mm for most of my panniers and aluminum fuel tank. I guess what you have is fine, but it's a little thick.
I'm noticing you have an inverter TIG, I used to have one similar now I have a transformer TIG.
I won't go into details about tungstens etc as I assume you have that down and I also have no idea what a metric size would be. I use 3/32" green (pure) tungsten with a flat ground, 5cf argon. I don't pretend to know the exact setup.
Here's what I can explain from noob to noob about how aluminum welds. You know when you get a puddle on steel? How you can floor it and the puddle is localized?
Well, that isn't happening with aluminum. It basically acts like a large heat soak so the whole part gets hot, then all of a sudden you get a puddle and if you aren't ready, you blow a hole right through it. Your thicker aluminum will help reduce this.
You need to be ready to back off the heat, when you see the puddle start to sink, you've got too much heat. I wait until the puddle gets shiny then back off the heat.
Back to your inverter machine. If it has pulse, you can use it to reduce heat into the workpiece. It also helps me with timing of dropping in filler material. Make sure you practice on unimportant parts first.
Use a stainless steel brush that you use for nothing else and keep it clean. Scrub the weld areas well until you remove the oxidized layer. I like the small ones like this:
Then I clean the surface with acetone. I also use the same acetone rag to wipe down the filler rods.
I used to use 4043 filler but now I use 5356 as someone told me that 4043 will look like crap if its anodized but the 5356 looks good.
Make sure you back way off the heat when you come to the end of a joint or you'll blow right through it. One thing I started doing is using a square steel backer for the joint as it help keep the metal where I want. You can also weld the back of the joint as a fillet weld and then weld the seams.
Anyways, I have no formal training, I basically asked questions and taught myself. I'm by no means a professional and wouldn't last a day as a welder, but I can do it well enough for what I want to do.